In a rear-projection type display device that projects imaging light on a screen from its rear, a transmission type screen has been used as a screen for displaying enlarged imaging light projected thereon. Generally, this transmission type screen has been used by combining an optical deflecting sheet (e.g., Fresnel lens sheet) that collimates imaging light projected from a light source by deflecting the same in parallel light rays, and emits the light to an observation surface side, and an optical diffusing sheet (e.g., lenticular lens sheet) that diffuses imaging light.
The optical deflecting sheet or the optical diffusing sheet used in such a transmission type screen has been conventionally formed by laminating an optical element such as a Fresnel lens or a lenticular lens on a plastic substrate made of, e.g., an acryl resin and a polycarbonate resin.
However, since the plastic substrate is sensitive to changes in the environment typified by temperature, humidity, and so on, the thickness and the size of the substrate tend to vary in accordance with changes in the environment. Thus, there may be a case in which the sheet is warped so as to generate a float (clearance) between the Fresnel lens sheet and the lenticular lens sheet. When imaging light is projected on the transmission type screen under such conditions, there arises a problem of significant decrease in an image quality, such as defocusing, a deterioration in reproducing colors, a double image, and a distorted image. In particular, when the transmission type screen is used in a thin rear-projection television, an angle of incidence of imaging light incident on the transmission type screen is large. Thus, only a small warp or a float generated in the screen considerably deteriorates an image quality of an image projected on the screen. In addition, as compared with a small-sized screen, a large-sized transmission type screen involves a problem in that deterioration in an image caused by a warp or a float, which has been generated by changes in the environment, is more conspicuous. At the same time, the large-sized transmission type screen is more likely to be inflected because of its own weight.
Therefore, a transmission type screen has been developed, that uses a glass substrate which is not easily warped under the influence of changes in the environment such as temperature and humidity.
JP2002-357868A discloses a transmission type screen that includes, in combination, a Fresnel lens sheet (described as “Fresnel lens plate” in the document), and a lenticular lens sheet (described as “lenticular lens plate” in the documents) having a lenticular lens (described as “lenticular lens sheet” in the document) laminated on one surface of a glass substrate. However, this transmission type screen has a problem in that, since a frontmost surface (surface closest to an observation side) of an emergent side of the transmission type screen is provided by the glass substrate, if the glass substrate is damaged by an accident or the like, pieces of broken glass are scattered. In this transmission type screen, mixture of a diffusing agent in the glass substrate is illustrated by way of an example, in order to impart a diffusing effect to the glass substrate. However, there exist the following problems. That is to say, it is not easy to mix a diffusing agent in a glass substrate, which increases the manufacturing cost of the glass substrate. In addition, mixture of a diffusing agent undesirably makes the glass substrate fragile, whereby the glass substrate is liable to be broken.
Besides, JP2001-154274A discloses a structure in which a glass substrate interposed between a lenticular lens sheet and a Fresnel lens sheet is tightly supported by the sheets that have been previously curved in opposite directions by means of a heat treatment. The method disclosed in this Patent Document 2 can be embodied, with use of a Fresnel lens sheet having a Fresnel lens disposed on its incident side. However, when a Fresnel lens sheet having a Fresnel lens disposed on its emergent side (glass-substrate side) is used, this is disadvantageous in that the Fresnel lens is rubbed by the glass substrate which tightly contacts a surface on which the Fresnel lens is disposed.
Further, JP2-183241A discloses a transmission type screen that includes a glass substrate with a Fresnel lens directly formed on one surface thereof, and a plastic sheet with a lenticular lens formed thereon, wherein the plastic sheet is integrally laminated on the other surface of the glass substrate. However, there are the following problems. That is to say, it is not technically easy to directly form a Fresnel lens on a glass substrate, which leads to increase in the manufacturing cost of the glass substrate. Further, it is only one surface of the glass substrate that is protected by the plastic sheet, while the opposite surface of the glass substrate with the Fresnel lens formed thereon is not protected. Thus, if the glass substrate is broken by an accident or the like, pieces of the broken glass substrate are scattered, or the Fresnel lens is scratched or stained by the pieces. Furthermore, there is another problem in that, since a concave portion of the Fresnel lens is concentrically subjected to a stress, the Fresnel lens can be easily broken.
Furthermore, JP2-42401A discloses a screen that includes: a plate-like Fresnel lens sheet having a lens layer attached to a surface of a glass substrate via an ultraviolet curing resin, and a protective layer of an ultraviolet curing resin layer formed on a surface of the lens layer; and another glass substrate attached on the protective layer. However, in the screen disclosed in this document wherein a frontmost surface is provided by the glass substrate, there is a problem in that, if the glass substrate is broken, pieces of the broken glass substrate are scattered.